Manufacturers of products ranging from aircraft and automobiles to home appliances share the need for effective means to cut (tap) female screw threads. Most commonly these threads are generated with threading taps, this is almost exclusively so in the case of small diameter threads. The general design of these taps is well established and their functional purpose well known to those skilled in the art of machining.
The technique for quickly and efficiently tapping large numbers of female threads on an automatic machine tool differs from the technique used by a craftsman charged with the task of tapping threads individually using manual methods. The craftsman can drill and tap a workpiece using a single spindle without disturbing the workpiece on the table of his drill press. He can be sure, therefore, that the tap enters the drilled hole squarely and precisely on center. Automatic machines, on the other hand, either move the workpiece from one station (tap drilling) to another (tapping), or bring spindles successively into position first to drill and then tap a fixed workpiece.
Incident to the technique of moving the workpiece, or shifting spindles between operations, is the problem of centering the tap precisely on the drilled hole. Despite high standards of machine tool accuracy, some degree of position error is unavoidable: if the error is small it will be expressed as a lopsided thread, cut heavily on one side of the hole. Greater positioning error results in tap breakage, the probability of damage to the machine or workpiece and certain interruption of machine operation.
To deal with the positioning errors in machine tapping, "floating" toolholders were developed. Floating holders enable taps with a "lead" (the nose of the tap in the form of a truncated cone) to enter a drilled hole as the tap is advanced (fed) toward the workpiece, to recenter on the drilled hole and thereafter continue to rotate on the hole centerline as threading proceeds. The floating toolholder accommodates radial displacement of the tap centerline from the centerline of the driving spindle in which the shank is installed. Torsional rigidity and axial rigidity in compression are maintained so that the tap may be forcefully fed into the workpiece and so the device can resist torque loads imposed on the tap by thread cutting.
Prior art floating holders function by permitting translation (sliding) of the head simultaneously along two radial axes displaced by 90.degree.. An intermediate coupling element that connects the shank (driving end) to the head (which holds the tap) carries the sliding elements. One popular design features a tongue and groove joint on each mating face of the coupling; another mounts radially disposed pins free to slide in radial holes in the head and shank.
Whereas these prior art toolholders serve their intended purpose, nevertheless they have several shortcomings. They are relatively complex, therefore, expensive. They are prone to breakage as many of their rather delicate small parts must be heat treated to high hardness (at the expense of toughness) to enhance wear resistance. Most holders can be used only with the tap size for which they were intended, others require separate adapter bushings. Current designs are prone to accumulating chips and grit that detract from the float, which, if left untended, eventually jam the unit completely.